The most obvious benefit of the hollow glass microsphere is its potential to reduce part weight, which is a function of density. Compared to traditional min-eral-based additives, such as calcium carbonate, gypsum, mica, silica and talc, hollow glass microspheres have much lower densities. For example, at a density of 0.6 g/cc, Sphericel hollow glass microspheres from Potters Industries (Valley Forge, Pa.), an affiliate of PQ Corp., can displace the same volume as talc at one-quarter the weight. Densities and crush ratings, however, vary dramatically across product lines.
“The density of the sphere will have a huge impact on the formulation of the part,” says Rosenbusch. Typical loadings are 1 to 5 percent by weight, which can equate to 25 percent or more by volume. For example, Potters’ lightweight Q-Cell hollow glass microspheres have a density (from 0.14 to 0.20 g/cc) approximately one-fifth that of most thermosetting resins. Therefore, on an equal weight basis, Q-Cell spheres occupy about five times more volume than the resin, which can reduce compound weight, VOC content and cost.
Historically, crush strength for hollow glass microspheres has been directly linked to density — i.e., a glass sphere with a density of 0.125 g/cc would be rated at 250 psi (1.8 MPa), while one with a density of 0.60 g/cc would be rated at 18,000 psi (124 MPa). To some degree, there remains a correlation.
The density and crush strength of hollow glass microspheres made from a particular material will depend, in part, on two structural variables, wall thickness and particle size.